Tom Cruise, pictured after delivering a speech at the opening of a Scientology church. Photograph: Paul Hanna/Reuters

Plans to broadcast HBO’s Church of Scientology exposé, Going Clear, have been shelved by Sky Atlantic in a virtual repeat of events two years ago, when UK publishers abandoned publication of the book on which the hard-hitting new TV documentary is based.

Sky originally indicated that the Alex Gibney-directed film, which alleges abusive practices at the religion’s US headquarters, would be transmitted in the UK earlier this month in step with its American release.

However, the Observer has learned that because Northern Ireland is not subject to the 2013 Defamation Act, the broadcaster could be exposed to libel claims from David Miscavige, the leader of the church, or others. This appears to have caused the company to postpone transmission, if not to cancel it entirely.

For technical reasons, Sky is unable to differentiate its signal between regions, rendering the same programme potentially exposed to pre-reform libel laws in Northern Ireland, but shielded in Britain where, among free-speech safeguards and reforms designed to limit frivolous claims or “libel tourism”, people or organisations must now show “serious harm” to reputation. Sky called the decision a delay rather than a cancellation, but did not deny it was taken for legal reasons. “At present, Sky’s transmission date for Going Clear has not been confirmed,” a spokesman said.

From its Los Angeles HQ, the church has denounced the film as a “one-sided, bigoted propaganda built on falsehoods” and informed by former members –whom it calls “misfits”. It is understood to have instructed solicitors in the UK to warn Sky it faces legal action if Going Clear is broadcast.

The church said in a statement: “The Church of Scientology will be entitled to seek the protection of both UK and Irish libel laws in the event that any false or defamatory content in this film is broadcast within these jurisdictions.”

Paul Tweed, a Belfast libel lawyer who represents Miscavige, was involved in previous correspondence with UK publisher Transworld, which ultimately dropped Going Clear, the Lawrence Wright book on which the documentary is based. Tweed argues against any change to existing libel laws. He considers Northern Ireland’s unreformed law the fairest in the world. He warns of “consequences of dissemination in jurisdictions where reputations can be protected”.

He added: “If Sky makes a decision to broadcast they will be making a commercial decision for commercial reasons. Any client of mine, whether a journalist or a high-profile personality, is entitled to protect their reputation against false allegations.

“In terms of Sky’s decision … they have to make absolutely certain that what they are broadcasting is not defamatory or untrue. If they have right on their side, they have nothing to fear.”

UK libel lawyers with no connection to the church, current or former members, say uneven implementation of the reforms across jurisdictions is causing chaos for broadcasters.

“It’s a headache because it’s quite difficult technologically to broadcast in just one jurisdiction,” says Gavin Millar QC of Matrix Law chambers in London. “Broadcasters can get sucked into litigation in Northern Ireland that they wouldn’t get sucked into in Britain.”

A programme broadcast in Northern Ireland on a controversial religion could attract a substantial number of viewers, raising exposure. “It just depends how important you think it is to broadcast in that jurisdiction, and how many people you think are going to see it.”

In the US, Going Clear has been seen by more than 5.5 million people and is likely to become second only to a film about Beyoncé as HBO’s most-watched documentary of the past decade.

Before its US premiere, Scientology took out a full-page ad in the New York Times questioning if Going Clear would turn out to be a story the film-makers and broadcaster would be forced to retract. The documentary claims to be an account of the church’s origins in the imagination of science fiction writer L Ron Hubbard and its development into an international organisation with billions of dollars in global property holdings. But interviews with eight former Scientologists about their experiences inside the church allege darker practices. Some claim they have been physically harassed and publicly vilified for speaking out. The church denies the allegations.

In a comment piece in the Los Angeles Timeslast week, Gibney claims: “In the past, critics of the church have called for its tax exemption to be revoked because it is not a ‘real religion’. I agree that tax-exemption isn’t merited, but not for that reason. The Church of Scientology has a distinct belief system which, despite its somewhat strange cosmology – mocked by the TV show South Park and many others – is not essentially more strange than, say, the idea of a virgin birth.

“Scientologists are entitled to believe what they want to believe. And the IRS [Internal Revenue Service] website makes it clear anyone is entitled to start a religion without seeking IRS permission. To maintain the right to be tax-exempt, however, religions must fulfil certain requirements for charitable organisations. For example, they may not ‘serve the private interests of any individual’ and/or ‘the organisation’s purposes and activities may not be illegal or violate fundamental public policy’.”

Regarding “private interests”, Gibney suggests Scientology is ruled by only one man, David Miscavige. Further, he claims “powerful celebrities within the church, particularly film star Tom Cruise, receive private benefits through the exploitation of low-wage labour and other use of church assets”.

While Cruise has not responded to allegations, fellow member John Travolta told the Tampa Bay Times the church “has been nothing but brilliant for me”. Travolta, who has no plans to see the documentary, called it “decidedly negative … I’ve been so happy with my [Scientology] experience in the last 40 years. I’ve been brought through storms that were insurmountable, and Scientology has been so beautiful for me, that I can’t even imagine attacking it.”

Yet former members continue to claim they are tracked and subjected to low-level harassment. Last week, the anti-Scientology writer Tony Ortega reported claims on his website, The Underground Bunker, that former member and outspoken critic Paul Haggis had been approached by a member of the church posing as a Time magazine reporter. The church has previously denied similar allegations.

Whether Sky will ever broadcast Going Clear is an open question. Under UK libel law, a publisher might only have to prove that “the statement complained was, or formed part of, a statement on a matter of public interest and they reasonably believed that publishing it was in the public interest”. But with the Northern Ireland assembly blocking the reforms from entering common law, it’s an argument lawyers for the broadcaster may not get the opportunity to make.